Many materials including natural materials such as wood and synthetic materials, such as plastics, rubbers, paints, varnishes, adhesives, sealants and the like need to be protected against photochemical degradation when used outdoors or otherwise exposed to ultraviolet radiation (UVR) from the sun or when in the presence of artificial UVR sources. Ultraviolet (UV) light can initiate chemical reactions in natural and synthetic materials and products, that result in discoloration and loss of chemical and physical properties. UV stabilizers are added to the material or a coating applied to the surface to reduce the photochemical degradation. The types of light or UV stabilizers currently used are UV absorbers that act by shielding the material from ultraviolet light or hindered amine (or amid) light stabilizers (HALS) that act by scavenging the radical intermediates formed in the photo-oxidation process. Often hindered amines and UV absorbers are used together to provide a level of stability which is higher than would be provided by using either type of stabilizer by itself. It has been reported that the most effective screeners are those with the highest and broadest absorbance in both the UVB (290-320 nm) and UVA (320-400 nm) ranges of the UV spectrum.
Currently both organic and inorganic UV screeners used in plastics, paints, varnishes and other materials are commercially available. Organic UV absorbers are used typically at 1 to 3% of binder solids, depending on coating thickness. Examples of organic UV absorbers include hydroxyphenyl-benzotriazol, benzophenone and hydroxyphenyl-triazine. In the case of very prolonged exposure to UV radiation (sunlight or light from artificial sources), however, organic UV absorbers slowly degrade and so lose their protective effect. The effects of weathering (humidity, high temperatures) and the like may cause a loss of UV absorber through diffusion and leaching.
Inorganic UV absorbers, such as, for example, titanium dioxide, cerium dioxide or zinc oxide do not photochemically degrade. Such inorganic particles have been used to provide more scratch-resistance and UV-protection in transparent coatings. The use of large alumina particles can cause the transparent coating to exhibit an undesirable hazy appearance. Because of the high surface area of titanium dioxide, cerium dioxide and zinc oxide nano-particles and the potential for photochemically induced reactions, there is possible photochemical damage and degradation of the organic matrix surrounding the inorganic UV absorbers. This can result in loss of adhesion between the coating and substrate. This degradation must then be addressed by, for example, using inorganic binders.